Mercury is listed third on the Environmental Protection Agency (EPA) list of toxic substances. According to the Mercury Study Report to Congress, coal combustion is the primary source of emissions in the United States, accounting for 72 percent of the 158 tons per year of the total mercury emissions.
Mercury is released from flue gas emitted from various types of plant stacks, and can exist in the environment in the form of elemental mercury, inorganic mercury salts or oxides, or organomercury compounds. Mercury in the air can then deposit in soil or water. The lifetime of elemental mercury in the atmosphere is a significant problem because it can reside in there for a period up to one year.
Because of the significant adverse effects of mercury accumulation in the ecosystem, stricter regulations regarding mercury emissions have been put in place. The regulation of hazardous air pollutants, including mercury, was significantly revised by Congress in the 1990 Clean Air Act. In December of 2000, the EPA announced that it will regulate mercury emissions from coal-fired boilers. Further, in January of 2004, the EPA issued their proposed rules in the “National Emissions Standards for Hazardous Air Pollutants” (40 CFR Parts 60 and 63). Through this new program, the EPA plans to reduce mercury emissions by nearly 70% by 2018. To comply with a consent agreement, the EPA also proposed a modest Maximum Achievable Control Technology standard to reduce mercury emissions by 29% by the end of 2007.
The development of mercury control technologies has been ongoing for the last decade. The implementation of EPA regulated controls for coal-fired power plants is scheduled for December 2007, yet there is currently no single best technology that can be applied broadly yet still efficiently remove mercury from the flue gas stream.
Further, based on what is readily available to date, mercury control technologies will be very costly, with estimates for its removal as high as $0.004/kWH ($3-$7 billion/yr). The primary problem for mercury emission control is that existing methods are highly dependent upon its speciation, or the form that the mercury is in. Specifically, most oxidized mercury compounds, including mercury oxide and mercury chloride, are more condensable and water soluble than other forms of mercury and, therefore, can be effectively captured in conventional pollution control systems (e.g., wet scrubber and particulate control devices).
Unfortunately, a significant portion of mercury, ranging from 30% to 70%, leaving the boiler is in its elemental form and therefore cannot be easily or efficiently captured. Although the injection of sorbents into the gas stream can enhance the capture of oxidized mercury through wet scrubbers, the performance of sorbents for elemental mercury capture is very limited. Further, if high amounts of sorbents are used in packed-bed adsorption facilities, it is expected that the cost will be even higher, because of the poor adsorption of elemental mercury by sorbents.
Two of the biggest concerns with mercury removal to date are the addition of supplemental substances such as chlorine to promote oxidation that can cause corrosion problems; and preventing the catalyst surface from becoming coated with solid particles, because as the particulate matter begins to coat the surface of the catalyst, the active surface area of the catalyst is reduced and therefore eventually rendered ineffective.
Therefore, if mercury control targets are to be met yet still be cost effective, new methods must be developed to easily and effectively remove mercury from flue gases emitted by coal-fired power plants.